Cellulosic materials, such as paper, wood, rayon and cotton are highly flammable substances and when ignited, the fire spreads rapidly. Many methods for retarding the spread of such fires are known, but for many commercial uses, such methods are often too costly or ineffective or render the physical characteristics of the flame retarding product undesirable.
Cellulosic materials used throughout industry and the home are closely associated with humans. Consequently, methods of chemically treating them to resist burning must assure a safe living environment and preserve their aesthetic qualities, particularly when the product is wood. The use of volatile or harmful solvents should be minimized during the process for treating the product with a chemical flame retarding agent, especially when a residue of such a solvent would exude from the processed material. The chemical treatment also should not produce or leach toxid or ugly deposits onto the surface of the material and must also maintain the aesthetic qualities of the material through the broad range of environmental conditions it may be subjected.
Chemical methods for flame retarding treatment of ligno cellulosic material, such as plywood panels, furniture wood, cardboard, paper and the like, include the application of salts of phosphoric acid, boric acid, sulfamic acid and the like as well as various organic compounds in most instances in organic solvent systems. These methods are economically attractive but cause crystallization of salts on the surfaces of the treated wooden product after drying, and cause the migration of undesirable chemicals to the surfaces. In addition to these difficulties, the solvents used are volatile during the application of the flame retardants and exude from the finished product.
A process for treating cellulosic material so that it will resist burning has been described in U.S. Pat. No. 3,900,327. The flame retardant is a reaction product of an aqueous orthophosphoric acid or an acid ammonium salt thereof and an alkylene oxide. The product is obtained by reacting an aqueous solution of an orthophosphoric acid or an acid ammonium salt thereof containing from about 1 to about 85 percent by weight of orthophosphoric acid or the equivalent amount of the ammonium salt thereof with an alkylene oxide, ethylene oxide being specifically exemplified, in an amount of from about 0.5 to 1.5 times the weight of the orthophosphoric acid or equivalent thereof used. This flame retardant has proven to be environmentally attractive as it utilizes an aqueous solvent which is substantially non-volatile. It nevertheless has several industrial disadvantages. When used in ligno cellulosic materials, it has difficulty meeting the Class II fire spread standard. Its rate of penetration into ligno cellulosic materials is too slow for the high speed equipment commonly used in the paper and wood panel industry. The cost of the treating chemicals, though lower than the majority of alternate chemical fire retardants, is sufficiently high as to cause an undesirable pricing of the finished product. In commercial processing, it has tended to produce crystals on the surfaces of the treated material, although this tendency is significantly reduced from other treatment processes; and, in highly humid atmospheres, it causes unattractive blotching rendering the finished product aesthetically and therefore commercially unacceptable.
Accordingly, a need exists for an economical, effective process and composition which will retard the spread of a fire in ligno cellulosic materials. The process should effect rapid penetration of the flame retardant composition through the surface of the cellulosic material and the composition should not deleteriously affect the surface characteristics of the treated product under a wide range of environmental conditions.